1. Field of the Invention
This invention relates to lithium ion secondary batteries and particularly relates to cylindrical lithium ion secondary batteries using a negative-electrode active material capable of forming an alloy with lithium.
2. Description of Related Arts
In recent years, power consumption of mobile devices including cellular phones, notebook computers and personal digital assistants (PDAs) has rapidly increased. Along with this, there has been an increasing demand to increase the capacity of lithium ion secondary batteries. However, if a graphite material, which has been widely used so far, is used as a negative-electrode active material for a lithium ion secondary battery, a problem arises in that it is difficult to sufficiently increase the capacity of the lithium ion secondary battery. In view of this problem, studies have been actively conducted on negative-electrode active materials having higher capacities than graphite materials.
Typical examples of new negative-electrode active materials currently proposed include materials capable of forming an alloy with lithium, such as silicon, germanium and tin. Among these materials, silicon, as for example described in Published Japanese Patent Application No. 2008-243661, has a high theoretical capacity of approximately 4000 mAh per gram. Therefore, much attention has been focused on silicon and silicon alloys as negative-electrode active materials that can achieve higher battery capacity.
However, negative-electrode active materials capable of forming an alloy with lithium, such as silicon, change the volume with storage and release of lithium. Therefore, if a cylindrical battery having a substantially unchanged volume is produced using a negative-electrode active material capable of forming an alloy with lithium, a volume change of the negative-electrode active material may cause a bend or the like in a central region of the spirally wound electrode assembly. If a bend or the like occurs in the electrode assembly, the separator may be damaged and the positive and negative electrodes may be electrically short-circuited.
In view of these problems, for example, Published Japanese Patent Applications Nos. 2003-308873 and 2004-273153 propose to insert a center pin into the center of the spirally wound electrode assembly. Published Japanese Patent Application No. 2003-308873 describes that the provision of such a center pin prevents the deformation of the electrode assembly, resulting in effective prevention of electrical short circuit between the positive and negative electrodes.
However, if a center pin is inserted into the center of the spirally wound electrode assembly, the space in the battery container not occupied by the battery components is reduced in volume. In other words, the allowable room for expansion of the spirally wound electrode assembly is reduced. Thus, the expansion of the spirally wound electrode assembly is prevented. As a result, the stress in the electrode assembly is increased, which may cause the separator to clog or cause the electrolytic solution to be extruded from the electrode assembly. Hence, if a center pin is inserted into the center of the spirally wound electrode assembly, a good charge/discharge characteristic may not be provided.
In order to avoid the prevention of expansion of the spirally wound electrode assembly, it can be considered, for example, to reduce the outer diameter of the center pin to prevent the center pin from coming into contact with the inner periphery of the spirally wound electrode assembly. For example, Published Japanese Patent Application No. 2003-308873 describes that the C/D ratio of the inner diameter (C) of the spirally wound electrode assembly to the outer diameter (D) of the center pin is selected to be 1:0.95 to 1:0.79.
However, if the center pin does not come into contact with the inner periphery of the spirally wound electrode assembly, the spirally wound electrode assembly may not suitably urge the center pin upon expansion of the negative-electrode active material. Thus, the center pin cannot sufficiently prevent the deformation of the spirally wound electrode assembly, resulting in electrical short circuit between the positive and negative electrodes.